Manually operated dispensers that dispense liquid as a foam are known in the prior art. One of these types of dispensers is a trigger sprayer that pumps liquid from a bottle attached to the trigger sprayer and discharges the liquid as foam. To produce denser foam from a liquid dispenser typically requires that both the liquid and air being mixed by the dispenser be under pressure. This generally means that the foaming dispenser includes both a liquid pump chamber and an air pump chamber. Typically, one or more pistons move between the charge and discharge positions in the air pump chamber and the liquid pump chamber to draw air or liquid into the respective chamber and force the air or the liquid from the chamber.
However, existing foam dispensers often require a number of complex components, multiple pistons, or elaborate passageways within the device. Furthermore, they often require complex check valve mechanisms to ensure proper flow of the liquid and air throughout the device. Additionally, some existing devices rely only on pressure differentials to operate the check valve at the outlet of the liquid pump chamber. So what is needed is a foam producing pump having a simple design utilizing a single piston. What is also needed is a foam pump having positive contact to help unseat the check valve at the outlet of the liquid pump chamber.